Praseodymium polyhydrides synthesized at high temperatures and pressures

Rare earth element polyhydrides have been predicted to exhibit high-Tc superconductivity at extreme compressions. Through a series of in situ high-pressure high-temperature x-ray powder diffraction experiments combined with density functional theory calculations, we report the emergence of polyhydride species in the praseodymium-hydrogen system. We initially observe the formation of PrH3, which continuously increases in hydrogen content on compression towards PrH4. Laser heating PrH4 in a hydrogen medium at pressures of 85 GPa leads to the synthesis of both PrH9 and PrH7. Both structures are characterized by hexagonal arrays of praseodymium atoms surrounded by hydrogen clathrate cages.